As suggested in this thread in the Python porting list (http://mail.python.org/pipermail/python-porting/2011-September/000231.html), it would be nice if 2to3 had a fixer that translated shebang lines from 

#! /usr/bin/env python

to

#! /usr/bin/env python3

Also relevant here is the draft PEP 394 (http://www.python.org/dev/peps/pep-0394/), which apparently is likely to be accepted.

"nice to have" isn't much of a requirement in my book.  The Python launcher for Windows http://legacy.python.org/dev/peps/pep-0397/ is also relevant.